Several fortification cards from L5R should map to this format easily. I'll convert a small set for castle building

. Fortifications are already in Fantasy Craft as an aspect of holdings...
NEW PRIZESBattlefield Fortifications While many locations include fortifications as a matter of course (it’s a dangerous world after all) Player Characters must spend Reputation to enhance their holding with such defensive measures. Battlefield Fortifications are an expansion to the holding rules, and do not take up an additional prize slot.
If a battlefield confrontation erupts at a fortified holding, the defending commander may distribute any fortifications the location possess amongst fronts the battle will have, doing so before the attacking commander chooses which fronts he will commit leaders to. If the location is owned by an NPC, the GM should perform this distribution in a fashion reflecting the situation. In battles with multiple fronts, only the fortifications at that front provide a benefit (unless otherwise stated).
You may purchase fortification with Reputation, selecting defensive features from table 2. You may acquire new fortifications (either replacing losses or exchanging existing construction with new selections) at the rate of 10 points per month of downtime. There are three limitations to adding battlefield fortifications to your holdings:
• You may only choose fortification with a Glory equal to or less than the holding’s fortification challenges
(see FC, page 193).
• You may choose a maximum number of fortification equal to that holding’s Scale
(see FC, page 192).
• You may not choose the same fortification more than twice per location.
A character may have fortifications at each holding he controls.
Common FortificationsConnecting Walls: The defending army gains a +2 gear bonus to their Battlefield checks at all fronts.
Dragons’ Teeth: Each defending unit gains +1 Force. Each attacking cavalry unit has -1 Force.
Moat: Each attacking special character and unit has -1 Force.
Pitch and Fire: You gain a trick.
Burning Oil Barrage 3+3 (ranged battlefield trick): Destroy two enemy units of your choice with Force 3 or less or a single unit with a Force of 6 or less. You may use this trick until you have succeeded with it once per battle.
Trenches: If the defending army loses it is dispersed, but no additional units are destroyed.
Tunnel System: Once per battle one leader and all his units unit may escape (leaving the battlefield).
Unscalable Walls: The defending army gains a +4 gear bonus to their Battlefield checks.
Table 2: Common FortificationsFortification Cost Glory Notes
Connecting Walls 8 4 +2 rolls, all fronts
Dragon’s Teeth 4 2 +1F defending units, -1F attacking cavalry
Moat 10 5 -1F for all attackers
Pitch and Fire 4 3 Burn!
Trenches 0 3 Messy escape
Tunnel System 4 2 Escape route
Unscalable Walls 6 5 +4 rolls